They want to scare/terrorize everyone into submitting to their ideas and beliefs, at any price, because they are lunatic extremists, whichever 'they' they belong to.
It's a really stupid and counterproductive idea that appears to appeal to any radical extremist of more or less any sort. If it's tried to sufficiently intense and lethal extent, it induces a period of repression that spreads a lot wider than them*. They -think- that will then induce massive widespread reaction/rebellion, but invariably it actually is widely welcomed because both the general population and their own subgroup feels unsafe & willing to sacrifice some freedom and choice in order to get more safety.
*See the German RAF, and the Italian Red Brigades, for classic examples of how this plays out.